Frequency selective apparatus



Jan. 25, 1949. w, F, KANNENBE 2,460,090

FREQUENCY SELECTIVE APPARATUS Filed Nov. 26, 1945 2 Sheets-Sheet 1 F 2, L 1- 2 D .137" F4- 5, L4-:; D= 3.363"

. lNl EN TOR WEKANNENBERG A 7' TORNE V Pie. 2.

Jan. 25, 1949.

Filed Nov. 26, 1945 w. F. KANNENBERG 2,460,090

FREQUENCY SELECTIVE APPARATUS 2 Sheets-Sheet lNl ENTOR 14 F KANNENBERG ATTORNEY Patented Jan. 25, 1949 UN TE I STATES M PATENT OFFICE f a aww i iwae v I A to Bell Telephone Laboratories, Incorporated,

New York N. Y.,', a 'co'rporation of New York 1 Application November 20, 1945, Serial No. 030,921

upon the configuration of the boundary surfaces of the resonator and upon theresonator dimensions; If itis desiredto utilize-the,resonator as a tunable selector of electrical oscillations the band over which tuning may be effected for a desired mode ofoscillation freeflgfrom interfer-. ence with oscillations .of an extraneous mode may be seriously restricted. It is possible to inhibit or suppress such interfering or extraneous modes of oscillations by various expedients which take advantage of an attenuation selective in its nature so as to discriminate strongly against oscillations of the unwanted mode .without presenting too serious attenuation tothe desired oscillations. However, if the unwanted oscillations and the desired oscillations are of the same general family as gmay be the case when both are transverse velectric modes, it is ordinarily considerably more .difiicult to effect such a selective discrimination. I

A particularly useful expedient applicable in designing cavity resonators of cylindrical type for various modes of oscillation is a straight line graph which portrays. the relationship between resonance frequency, diameter, and length of the resonator for a particular mode of interest. A chart including the graphs of the various modes of oscillations which mayfall within a. given frequency band may be utilized to secure results which it is otherwise quite difficult to attain. The derivation of the graphsof such a chart may begin with the resonance formula for aperfect cylinder. o q

. 2 2 a V V r TE K Where i is the frequency in cycles per second, D is the diameter of the cylinder in centimeters; L is the length of the cylinder in centimeters; 1

is the Bessel function r500 for the selected mode bs l fiqn i e filth?. ls ii easasi orepair '3. Claims. (01. 178-44) constant 2.998 1'0 'centimeters per second, and n is thesubscript oi the oscillation mode which denotes the number of half wavelengths in the direction of the longitudinal axis ofthe cylin der. Equation 1 maybe rew'ritt'enas A and K being constants; is evident this is the equation of va a s ra h 1. slfiile .1 D)? #9 Each oscillation mode of a :perfect, cylindrical resonator may be plotted as. a straight line and the possibilities for encountering interference and for avoiding that interference may thenibe readily ascertained. [Ii o In accordance with theinvent'ion a cavity reso-1 nator of generally cylindrical conformationais so designed that it is capable of havingthe ratio of its diameter to its length changed at will. The resonator may thenbe tuned in the usual manner as, for example; by moving an end plate until an intermediate frequencyds reached ad-. jacent the frequency of oscillations of a trouble-. some mode. By next changingthe efi'ective'diameter of the resonator its performance may be shifted so as to cause itto. passv over, the fre-v quency of the unwanted oscillation to. a point on the new graph which is also. the same as the intermediate frequency. Thereafter tuning may proceed: freeifrom interference bythe oscillations of the unwanted mode up to some point at which oscillations of another mode become troublesome. a v c .An expedient which may be-used for quickly changing the effective diameter and with it the ratio of diameter to. length of a cylindrical reso: nator comprises providing the resonator with a longitudinally tapered cylindrical shell and separately supported end 'plates. After. tuning in the usual manner by moving one of the end plates has proceeded over a first hand to a frequency adjacent that of oscillations of the troublesome interfering mode,. tuning is discontinued and the tapered cylinder is moved longitudinally with.

respect to the end plates sozthat the'resonator changes its effective average diameterthus hop again begin using a new scaleto sweep over a second band contiguous to the first band.

In the drawing Fig. 1 shows a chart of certain graphs portraying some resonances of a cavity resonator of cylindrical conformation;

Fig. 2 is a schematic diagram partly insec- 7 4 .7 P2 which corresponds to a length on the 111-2 scale of 11.945inches. Had we carried the tuning operation farther to 9413 megacycles at point Pz interference would have been encountered from the TE3,2,s mode, the graph of which intersects TEo,1,17 at point P3. The next step after tion of aicavity'reso'nator embodying the present invention;

' Fig. '3 is a section of the cavity resonator oi the plane 3-3, and

Fig. 4 is a section of the fixed end plate taken along the line44 of Fig. 3.

Equation 3 presents the "relations 1p between" the quantities D) 2 and Graphs may be plotted for'each modeof oscillation occurring within a particular range such as TEo,1,17, TE3,2,6 and Th-6,1,9, etc; Thus we may bitrary units which include the proportionality constants and the abscissae scale-of is applicable generally to all cylindrical strum tures: It is possible, however, to :plot resonator Q10 Fig. 2'looking in the direction of the arrows at" reaching the point P2 is to discontinue tuning and to decrease diameter of'the resonator to 3.363

inches. This reduction of '7mils in diameter, as

has already been explained, shifts the magnitudes of the frequency and length scales since it changes theratio'cfD to'L'upon which the frequency depends. Accordingly, the frequencies of all the points which we'havebeen considering on the graph oifFig. 1..are increased. For examplathe point P2 now corresponds to a length 11 .948 inches and now occursat 9440 megacycles as ascertained from the ordinate scale f4-5; The mode crossing or interference point P3 is at 9433 megacycles. It will be recalled that we had stopped the tuning operation at 942il megacyclesp In orderto resume tuning at thatfrequency; we first slightly readjust our tuner by suitable adjustments of the positicn of the-movable tuning piston tothe point P4 which, at a length 11.953 inches, corresponds to t the frequency 9420 on the f ra sca-l'e. B'ythis op erationof changing essentiallyoonly the tube diameter we 'have hopped over the interfering point P3 without having to use thatpoint in tuning. From a frequency of 9420 megacycles at point P4 we continue the tuning operation to the 937 3 megacycle point "P5 by increasing the :length L to 12.063 inches, which-is essentially the same length we would have used (125058) had we not changed tube diameter. t v i It will be clear that the 'in-vention "provides a technique enabling the interference-free tunable range 02 a cavityresonator for a given mode of oscillations to be greatly augmented-provided we ing 'the length 'of the resonator in inches when used in conjunction with the fi-z scale for'astru cture having a fixed diameter of 3.37 inches and;

thettl'n-s scale serving similarl-ycfor the range -el fi-s frequencies .for inches.

Suppose that we wish to use oscillations TEo,1,1'1 mode and that we desire to tune over a" band of frequencies oeginning with an upper frequency of 9463' megacyclesaridthat theeylim' drical resonator has a diameter of3437 inches. The" conditions are those of the point. P1 on'the TEtnm graph. The length {L' of :theresonator or the distance between its end plates obtainable from theLr-z scale-is 1-1.88inches. It, now,-we begin-the tuning operation by'causing the movable end'pla'te of the resonator to be shifted in a direction away from thefixed end plate, the length ofthe resonator increases-and the frequency falls. We-may-carry this operationthroughout an inter ference-free band-to some frequencysuch as 9420 megacycles at the intermediate "frequency point a fixed diameter of 3.363:

are able to change the diameter of the cavity resonator so'as tohop over a mode intersectionpoint. a e

One embodiment of a means for effecting change in diameter of a tunable resonator is shown in Fig. 2. Instead of a sudden transition from one diameter to another it relies uponthe change in the average-effective diameter'which is: obtained if the-cylindrical shell is made appre c'iably longerthan its used-portion andi is slightly-'- tapered-orconical so that its internal diameter increases from Do:at its smaller end to D0+AD6 at the larger end. Such; afltapered- :cylindrical plates 2! and 22,.the structure being mountedon a base member 23; Input and-output coaxial circuits extend. to narrow slot apertures 26- in the fixed plate 22 and terminateinenergytransfer loops. Ed -and iawhich extend into the internal electromagnetic field 'of the resonator 2-0 in planes tangentialto the circularly directed electric field vectors. The planes of these-loops'l'ie at approxi-- matel'y midpoints between the center and the peripheral boundary of the resonator 1 chamber at which positions the circularly directed electricf vectors are of substantially maximumintensity for; oscillations of TEo modes. The other end Z-l' of the resonator is supported by a toothed rack memher 2! slidably mounted on the base 23 in a track 23 which holdsrtheimovableftuning piston 2| with its center aligned with the longitudinal axis of the resonator shell 28. In order to move the pis-..

with the teeth oftherackmember '2! is provided:

The shaft 29' may be provided with any known hand operating mechanism, not shown. This permits the position of the piston 28 .to be adjusted inwardly or outwardly as desired. In like manner the tapered cylindrical shell 29 is mounted on a rack member 32 for movement in a longitudinal direction along the track 28. Movement of the resonator 2c is effected by means of a knob (not shown) attached to the shaft 33 which is mounted on the base plate 23 and carries a toothed pinion 34 engaging the teeth of rack 32.

The piston 25 and the fixed end plate 22 determine the length L of the resonator space which is measured by their separation. The diameter of the sh ll as is, as has been stated, slightly tapered. It may increase, for example, by an amount of 2 mils for each inch in longitudinal direction so that the difference in diameters of the end of the 12-inch tube may amount to 24 mils. The tapered cylinder may be a slightly tapered aluminum tube silver plated in the usual manner. It may also be a cylindrical tube having an inner silver plating which is varied in thickness from one end of the tube to the other to constitute a tapered bore. Among other suitable alternatives is a glass tube with a tapered bore and an interior plating or coating of elec trically conductive substance. I

L1 operation of the apparatus of Fig. 2 in ac cordance with the method which has been out lined in connection with Fig. 1, the tube 28 may be shifted to the left to a position such that its average effective diameter 3.37 inches. In order to accomplish this with considerable precision the resonator 25 may carry a springdetent 36 cooperating with notches 3? in a retainer fixedly attached'to the stationary track 23. At the beginning of the tuning operation the tuning piston 25 may occupy a position in which it is nearest to the fixed end plate 22. Tuning may proceed by operation of the shaft is to move the piston 2i to the left, the frequencies being indicated by an indicator pointer 33 moving in a direction to the left over the right-hand scale 39. When the intermediate frequency point P2 is reached and it becomes desirable to reduce the diameter the resonator 25 may be moved to the right thus reducing the efiective diameter of the cylindrical chamber between piston 2i and end plate 22 to a predetermined magnitude which may correspond to the position the detent 36 may occupy. The piston 2! may now be reset to such a point that the pointer 38 indicates the same intermediate frequency on the scale it as it had previously indicated on the scale 39. Thereafter tuning may continue by moving the piston 29 to the left.

It will be evident that this same procedure may be applied to avoid other mode crossings if it be desired to extend the range of the continuous tuning band for which the resonator may be free of interfering modes. For this purpose it will be necessary to increase the length of the resonator shell 20 so that its diameter may be d changed by additional steps by shifting the shell longitudinally. For these additional ranges scales H and 32 may be provided.

The fixed end plate 22 and the piston as well may consist of plates dielectric material '13 with metallic front plates dd and back plates it to provide against Warping and to present a highly electrically conductive surface as a boundary of the electromagnetic held. The peripheral gaps separating the end plate 222 and the piston 2i from the shell 2%! enhance the dis crimination which the resonator exhibits in favor of oscillations of TED mode and against those of modes involving radially directed electric vectors.

What is claimed is l. A cavity resonator comprising a conducting surface in the form of a tube having varying internal cross-sectional area, parallel end walls defining the boundaries of an electromagnetic field within the tube, means for moving one of the end walls with reference to the other to vary the tuning of the resonator over a band of fre quencies and means for moving the tubular conducting surface longitudinally relatively to the position of the end walls.

2. A cavity resonator comprising a conducting surface in the form of a irustum of a cone, parallel end walls defining the boundaries of an lectrornagnetic field therewithin, means for moving one of the end walls with reference to the other to vary the tuning of the resonator over a band of frequencies, means for moving the conical conducting surface longitudinally relatively to the position of the end walls and means for exciting oscillations of TE] mode within said resonator.

3. A cavity resonator comprising a conducting surface in the form of a irustuin of a cone, parallel end walls defining the boundaries of an electromagnetic field therewithin, means for moving one of the end walls with reference to the other to vary the tuning of the resonator over a band of frequencies, means for moving the conical conducting surface longitudinally relatively to the position of the end walls, means for exciting oscillations of TEn mode within said resonator, and a series of external scales so calibrated as jointly to enable a continuous tuning over contiguous bands without interference from an extraneous mode lying just outside one of said bands in the region of the other.

W'ALTER F. KANNENBERG.

REFERENCES CITED Ihe following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,281,550 Barrow May 5, 1942 2,315,313 Bucholz Mar. 30, 1943 2,405,277 Thompson Aug. 6, 1946 

